"止" means to stop. To break-up the word wu, it is "止" and 戈 (ge). "ge" is an ancient weapon, but used to descript conflict or fight in this context. There is a saying 大动干戈. Meaning, a big conflict or fight. So the word wu consist of the stop, and conflict or fight. As the saying goes, 止戈为武. The actual spirit of Material Art is not in the fighting or creating conflict but to stop it.